1. Field of the Invention
The present invention relates to a liquid crystal display device, and a method and a circuit for driving the liquid crystal display device. In particular, the present invention relates to a liquid crystal display device which can respond at high speed with high efficiency, and a method and a circuit for driving the liquid crystal display device.
2. Description of the Related Art
With the progression of the age of multimedia, various types of liquid crystal display devices, from a small one used in a projector device, a cellular phone, a viewfinder, and the like to a large one used in a notebook PC, a monitor, a television, and the like, have rapidly become widespread. A medium-sized liquid crystal display device has become essential in electronic equipment such as a viewer and a PDA, and in a game instrument such as a portable game machine and a pachinko (Japanese pinball game) machine. The liquid crystal display device has been used in various types of equipment down to a household electrical appliance such as a refrigerator and a microwave oven. Currently, almost all liquid crystal display elements are in a twisted nematic (hereinafter referred to as “TN”) type display device. The TN liquid crystal display element takes advantage of a nematic liquid crystal composition. When the conventional TN liquid crystal display element is driven by simple matrix drive, display quality is not high, and the number of scanning lines is limited. Thus, an STN (super twisted nematic) type device is mainly used in the simple matrix drive system, instead of the TN device. In the STN device, contrast and viewing angle dependence have been improved, as compared with an initial simple matrix drive system using the TN device. The STN liquid crystal display device, however, is not suited for displaying moving images because the response speed thereof is slow. To improve the display performance of the simple matrix drive, an active matrix device, in which each pixel is provided with a switching element, has been developed and widely used. For example, a TN-TFT device that uses a thin film transistor (TFT) in the TN type display has been generally used. The active matrix device using the TFT can realize higher display quality than the simple matrix drive, so that the TN-TFT liquid crystal display device has currently become the mainstream of a market.
In response to a demand for further improving image quality, on the other hand, a method for improving a viewing angle has been researched and developed, and in practical use. As a result, three types of active matrix liquid crystal display devices have become the mainstream of a current liquid crystal display with high performance. One of the three types is the TN LCD using a compensation film. Another is the TFT active matrix LCD in an IPS (in plane switching) mode, and the other is the TFT active matrix LCD in an MVA (multi-domain vertical aligned) mode.
In these active matrix liquid crystal display devices, positive and negative writing is generally carried out by using an image signal of 30 Hz. Thus, an image is rewritten every 60 Hz, and time for a single field is approximately 16.7 ms (milliseconds). Namely, the total time of positive and negative fields is called a single frame, and is approximately 33.3 ms. As compared with this, the response speed of current liquid crystal is on the order of this frame time even in a fastest condition, in consideration of a response during halftone display. Thus, when an image signal composed of moving images, high speed computer graphics (CG), or high speed game images is/are displayed, a response speed faster than the current frame time is necessary.
On the other hand, a current mainstream pixel size is approximately 100 ppi (pixel per inch), and pixels have been further fined by the following two methods. One of the methods is to reduce the pixel size by increasing the accuracy of processing. The other method is to adopt a field sequential (time division) color liquid crystal display device. In the field sequential (time division) color liquid crystal display device, a backlight serving as illumination light of the liquid crystal display device is switched among red, green, and blue in accordance with time. Red, green, and blue images are displayed in synchronization with the switching of the backlight. According to this method, it is unnecessary to spatially dispose a color filter. Thus, it is possible to improve the display resolution three times as fine as the conventional one. In the field sequential liquid crystal display device, since a single color has to be displayed for one-third time of the single field, time available for display is approximately 5 ms. Therefore, it is required that the liquid crystal itself respond faster than 5 ms.
From the necessity of such high speed liquid crystal, various technologies have been considered, and some of high speed display mode technologies have been developed. These technologies for the high speed liquid crystal are mainly divided in two trends. One is a technology for speeding up the foregoing nematic liquid crystal being the mainstream. The other is a technology for using a spontaneous polarization type of smectic liquid crystal that can respond at high speed, or the like. The speedup of the nematic liquid crystal, being a first trend, is mainly carried out by the following means. (1) Thinning a cell gap, and increasing electric field intensity at the same voltage. (2) Applying a high voltage, and increasing electric field intensity to accelerate change in a state (an overdrive method.) (3) Reducing viscosity. (4) Using a mode to be thought of high speed in principle.
The following problems occur in such high speed nematic liquid crystal. In the high speed nematic liquid crystal, a liquid crystal response is almost completed within the frame, so that variation in capacitance of a liquid crystal layer due to the anisotropy of permittivity becomes extremely large. The variation in the capacitance causes variation in a holding voltage to be written into and held in the liquid crystal layer. The variation in the holding voltage like this, that is, variation in an effective applied voltage lowers contrast due to a shortage of writing. When the same signal is written continuously, luminance keeps varying until the holding voltage stops varying, and hence several frames are necessary to obtain stable luminance.
To prevent such a response needing the several frames, it is necessary to provide a one-to-one correspondence between an applied signal voltage and obtained transmittance. In the active matrix drive, transmittance after a liquid crystal response is determined in accordance with the amount of electric charge accumulated in a liquid crystal capacitor after the liquid crystal response, instead of the applied signal voltage. This is because the active drive is a constant electric charge drive in which the held electric charge makes the liquid crystal respond. The amount of electric charge supplied from an active element is determined by accumulated electric charge before writing a predetermined signal and newly written electric charge, when omitting a minute leak and the like. The accumulated electric charge after the response of the liquid crystal varies in accordance with pixel design values of the liquid crystal such as physical constants, electric parameters, and storage capacitance. Therefore, to make the signal voltage and the transmittance correspond to each other, information for calculating (1) correspondence between the signal voltage and the written electric charge, (2) the accumulated electric charge before writing, and (3) the accumulated electric charge after the response, actual calculation for the items (1) to (3) and the like are necessary. As a result of this, a frame memory for storing information in the item (2) over the whole screen, and calculation sections for the items (1) and (3) become necessary.
On the other hand, a reset pulse method is often used as a method for establishing a one-to-one correspondence without using the foregoing frame memory and the calculation sections. In the reset pulse method, a reset voltage is applied before writing new data to align the liquid crystal in a predetermined state. By way of example, a technology disclosed in IDRC 1997 pages L-66 to L-69 will be described. The technology disclosed in this document uses an OCB (optically compensated birefringence) mode, in which nematic liquid crystal is in pi-alignment and a compensation film is added. The response speed of this liquid crystal mode is approximately 2 to 5 milliseconds, and is much faster than that of the conventional TN mode. As a result, a response which should be originally completed within a single frame needs several frames, as described above, until variation in permittivity by a response of the liquid crystal significantly decreases the holding voltage and stable transmittance is obtained. Thus, a method for necessarily writing black display after writing white display within the single frame is shown in FIG. 5 disclosed in the IDRC 1997 pages L-66 to L-69. This drawing is quoted as FIG. 1. Referring to FIG. 1, a horizontal axis represents time, and a vertical axis represents luminance. A dotted line that indicates variation in the luminance in the case of normal drive reaches the stable luminance at the third frame. According to this reset pulse method, since the liquid crystal is certainly in a predetermined state in writing new data, it was possible to establish the one-to-one correspondence between a written constant signal voltage and constant transmittance. The generation of a driving signal becomes extremely easy because of the one-to-one correspondence. Also, means for storing previously written information such as the frame memory becomes unnecessary.
The structure of a pixel of an active matrix type of liquid crystal display device will be hereinafter summarized. FIG. 2 shows an example of a pixel circuit of a single pixel of the conventional active matrix type of liquid crystal display device. As shown in FIG. 2, the pixel of the active matrix type of liquid crystal display device comprises a MOS transistor (Qn) (hereinafter called a transistor (Qn)) 904, a storage capacitor 906, and a liquid crystal 908. A gate electrode of the transistor (Qn) 904 is connected to a scan line (or a scan signal electrode) 901. One of source and drain electrodes of the transistor (Qn) 904 is connected to a signal line (or an image signal electrode) 902, and the other of the source and drain electrodes is connected to a pixel electrode 903. The storage capacitor 906 is formed between the pixel electrode 903 and a storage capacitor electrode 905. The liquid crystal 908 is disposed between the pixel electrode 903 and an opposed electrode (or a common electrode) Vcom 907.
Currently, in a notebook personal computer (notebook PC) which forms a large application market of the liquid crystal display device, an amorphous silicon thin-film transistor (hereinafter abbreviated as a-Si TFT) or a poly-silicon thin-film transistor (hereinafter abbreviated as p-Si TFT) has been generally used as the transistor (Qn) 904. As a material for the liquid crystal, a TN liquid crystal has been used. FIG. 3 shows an equivalent circuit of the TN liquid crystal. As shown in FIG. 3, the equivalent circuit of the TN liquid crystal comprises a capacitor component C3 of the liquid crystal (capacitance Cpix), a resistor R1 (resistance Rr), and a capacitor C1 (capacitance Cr). The capacitor component C3 is connected in parallel with the resistor R1 and the capacitor C1. In this equivalent circuit, the resistance Rr and the capacitance Cr are components for determining a response time constant of the liquid crystal.
FIG. 4 is a timing chart of a scan line voltage Vg, a signal line voltage (or image signal voltage) Vd, and a voltage Vpix of the pixel electrode 903 (hereinafter called a pixel voltage), in the case where such a TN liquid crystal is driven in the pixel circuit shown in FIG. 2. As shown in FIG. 4, since the scan line voltage Vg is at a high level VgH during a horizontal scan period, the n-type MOS transistor (Qn) 904 is turned on. Therefore, the signal line voltage Vd inputted into the signal line 902 is transferred to the pixel electrode 903 through the transistor (Qn) 904. The TN liquid crystal normally operates in a mode, in which light passes through when voltage is not applied, that is, the so-called normally white mode.
In FIG. 4, voltage for increasing transmittance through the TN liquid crystal is applied as the signal line voltage Vd over a few fields. When the horizontal scan period is completed, and the scan line voltage Vg becomes a low level, the transistor (Qn) 904 is turned into an off state. Thus, the signal line voltage transferred to the pixel electrode 903 is held by the storage capacitor 906 and the capacitor Cpix of the liquid crystal. At this time, the pixel voltage Vpix carries out a voltage shift, which is called a feed-through voltage, through capacitance between the gate and the source of the transistor (Qn) 904, at a time when the transistor (Qn) 904 is turned off. This voltage shift is indicated by Vf1, Vf2, and Vf3 in FIG. 4. Increasing a value of the storage capacitor 906 makes it possible to reduce the amount of the voltage shift Vf1 to Vf3.
The pixel voltage Vpix is held, until the scan line voltage Vg becomes the high level again in the next field period and the transistor (Qn) 904 is selected. The TN liquid crystal is switched in accordance with the held pixel voltage Vpix. Light transmitted through the liquid crystal shifts from a dark state to a bright state as shown in transmittance T1. At this time, as shown in FIG. 4, the pixel voltage Vpix varies by ΔV1, ΔV2, and ΔV3 in each field. This is because the capacitance of the liquid crystal varies in accordance with the response of the liquid crystal. To minimize this variation, the storage capacitor 906 is generally designed so as to have two, three times or more as large capacitance as the pixel capacitor Cpix. As described above, the TN liquid crystal is driven by the pixel circuit shown in FIG. 2.
Japanese National Publication No. 2001-506376 discloses technology for modulating a common voltage (common electrode voltage (or opposed electrode voltage)). The technology has the effects of a combination of the overdrive method and a reset method. FIG. 2C of this Publication No. 2001-506376 is quoted as FIG. 5. In this technology, the common voltage, being the voltage of a common electrode disposed opposite to the pixel electrode, is generally modulated. In FIG. 5, an upper graph indicates variation in the common voltage (VCG) with time, and a lower graph indicates variation in transmittance (I) with time due to a liquid crystal response. In other words, a voltage having a voltage waveform 151 is applied to the common electrode, and a light intensity waveform 152 indicates light intensity at time corresponding to the waveform 151. Line segments 153 to 156 are pixel light intensity curves. In technology prior to this technology, the common voltage was kept at constant during drive. Otherwise, common inversion drive, in which the common voltage was changed between two voltage values at regular intervals when each of periods of t0 to t2 and t2 to t4 of FIG. 5 was regarded as a single frame period, was carried out. In the Japanese National Publication No. 2001-506376, the single frame period is divided in two, and a voltage having approximately the same amplitude as that of the conventional common inversion drive is applied during each of periods from t1 to t2 and from t3 to t4. On the other hand, a voltage higher than the amplitude of common inversion, that is, for example, a voltage higher than the amplitude of the common inversion by a voltage applied for black display is applied during each of periods from t0 to t1 and from t2 to t3 in the single frame period. According to this technology, since the high voltage is applied to the common electrode during the period from t0 to t1, difference in voltage between the pixel electrode and the common electrode becomes large. Thus, it is possible to rapidly change the whole display area into the black display. In other words, drive corresponding to the reset drive is carried out. Furthermore, if image data is written into the pixel electrode during the period from t0 to t1, the image data is not observed in the display area because the difference in voltage between the pixel electrode and the common electrode is sufficiently large (for example, more than black display voltage). After the image data is written into the whole display area, the voltage of the common electrode is returned to the amplitude of the common inversion at the timing of t1. As a result, a liquid crystal layer starts responding to change transmittance corresponding to each gray level, in accordance with the voltage stored in the pixel electrode. Namely, the difference in voltage changes from a large value to a value corresponding to each gray-level voltage whenever a response starts. In this respect, a kind of overdrive is carried out during the period from t0 to t1.
Note that the response time of liquid crystal is generally expressed by the following two equations (refer to page 24 of “Liquid Crystal Dictionary” Baifukan Co., Ltd, edited by Japan Society for the Promotion of Science, 142th Committee on Organic Materials Used in Information Science and Industry, Liquid Crystal Division.) Namely, the following equation 1 is satisfied at a rising response (ON response), in which a voltage higher than a threshold voltage is applied to turn on the liquid crystal.
                              τ          rise                =                                            d              2                        ·                          η              ~                                            Δ            ⁢                                                  ⁢                          ɛ              ·                              (                                                      V                    2                                    -                                      V                    c                    2                                                  )                                                                        Equation        ⁢                                  ⁢        1            
The following equation 2 is satisfied at a falling response (OFF response), in which the applied voltage higher than the threshold voltage is abruptly lowered to zero.
                              τ          decay                =                                            d              2                        ·                          η              ~                                                          π              2                        ·                          K              ~                                                          Equation        ⁢                                  ⁢        2            
In the foregoing equations, “d” represents the thickness of a liquid crystal layer, “η” represents rotational viscosity, “Δ∈” represents dielectric anisotropy, “V” represents the applied voltage corresponding to each gray level, “Vc” represents the threshold voltage, and “K” represents a Frank elastic constant. The following equation 3 is satisfied in the TN mode.
                              K          ~                =                              K            11                    +                                    1              4                        ⁢                          (                                                K                  33                                -                                  2                  ·                                      K                    22                                                              )                                                          Equation        ⁢                                  ⁢        3            
In the foregoing equation, “K11” represents a splay elastic constant, “K22” represents a twist elastic constant, and “K33” represents a bend elastic constant. As is apparent from the equation 1, the response time of the liquid crystal is in proportion to the reciprocal of the square of the applied voltage at the rising response (ON response). Namely, the response time of the liquid crystal is in proportion to the reciprocal of the square of the applied voltage, which differs on a gray level basis. Thus, the response time largely differs in accordance with the gray level, and when voltage differs 10 times the response time differs 100 times. On the other hand, difference in the response time due to the gray level exists even in the falling response (OFF response), but the difference remains to the extent of double.
Note that the technology disclosed in the “Liquid Crystal Dictionary” (Baifukan Co., Ltd, edited by Japan Society for the Promotion of Science, 142th Committee on Organic Materials Used in Information Science and Industry, Liquid Crystal Division). The speed of the liquid crystal is increased at the rising response (ON response) by the effect of overdrive. In the overdrive, an extremely high voltage is applied. All responses used for displaying an actual image are the falling responses (OFF responses), so that they hardly depend on the gray level. Therefore, it is possible to obtain approximately the same response time over all gray levels.
The foregoing liquid crystal display devices, that is, the display device by the overdrive, the display device by the reset drive, the display device disclosed in a document such as Japanese National Publication No. 2001-506376, however, have several problems.
A first problem is that the rising response speed of the liquid crystal can be increased in the overdrive method, but the response speed is confined from several tens milliseconds to a dozen or so milliseconds under the constraint of a material. As to the falling response speed, it cannot be much increased.
This is explained as follows. To improve the response speed of the liquid crystal element itself, as is apparent from the equations 1 and 2, the following contrivances are effective:    (1) Thinning the width “d” of the liquid crystal layer;    (2) Reducing the viscosity “η;”    (3) Increasing the dielectric anisotropy “Δ∈” (only in the rising response);    (4) Increasing the applied voltage (only in the rising response); and    (5) Of the elastic constants, decreasing “K11” and “K33” and increasing “K22” (only in the falling response).In regard to (1), however, the thickness of the liquid crystal layer is variable only within the confines of constant relation with refractive index anisotropy “Δn,” in order to obtain a sufficient optical effect. Since all of the viscosity, dielectric anisotropy, and elastic constants of (2), (3), and (5) are physical values, they greatly depend on the material. Thus, it is difficult to increase/decrease the viscosity, dielectric anisotropy, and elastic constants to predetermined values or more/less. Furthermore, it is extremely difficult to largely change only each physical value itself, so that it is difficult to realize the effect of speedup assumed by the equations. For example, “K11,” “K22,” and “K33” are the independent elastic constants, but a relation of K11:K22:K33=10:5:14 approximately holes according to the measurement result of the actual material. Thus, “K11,” “K22,” and “K33” cannot be always treated as the independent constants. According to this relation and the equation 3, for example, K=11·K22=5, and only “K22” is independent. Therefore, improvement at a few tens percent or more is impossible, though slight adjustment is possible. A method of increasing the applied voltage value according to (4), on the other hand, receives severe constraint from the viewpoints of electric power consumption and the high cost of a high voltage driving circuit. At the same time, when the active element such as a thin-film transistor is provided in the display device and driven, the withstand voltage of the element adds constraints to the display device. As described above, there are severe limitations in speeding up the response speed by the conventional contrivances such as the overdrive.
A second problem is that the overdrive method can speed up the rising response (ON response), but hardly speed up the falling response (OFF response). This is because, as is apparent from the equations 1 and 2, the response time varies dependently on potential difference in the ON response, but the response time does not depend on the potential difference in the OFF response. As a result, in the conventional overdrive method, the OFF response dominantly determines the response speed of the whole system.
A third problem is that the voltage necessary for the overdrive is high in the conventional overdrive method. An image signal was a high frequency signal in the display device. In the overdrive method in which the voltage of the image signal was increased, increase in electric power consumption was significant. Since it was necessary to generate a signal with high frequency and high voltage, a drive IC and a signal processing system identical to conventional ones could not be used. Thus, an IC using specific process or an expensive IC had to be used.
A fourth problem is that in the reset method, a method for applying a reset signal through the pixel switch complicates the structure of a drive system and increases electric power consumption. Namely, it becomes necessary to drive scan lines differently from a scan for writing the image signal in terms of a scan period and a scan method. When the pixel switch is reset, a method for collectively resetting all the scan lines is often used instead of a successive scan. Therefore, structure for collectively sending a signal to the whole screen is necessary in the scan system. Driving the scan lines not only in writing the image signal but also in writing the reset signal causes increase in the frequency of a signal for a scan line, the voltage amplitude of which is the highest in the display device. Thus, the electric power consumption is increased. From these points of view, it is desirable that the reset not be carried out through the pixel switch.
A fifth problem is that a display state significantly changes in accordance with the redundancy or lack of reset in the reset method. This problem also goes for the method disclosed in the Japanese National Publication No. 2001-506376, which is the combination of the overdrive method and the reset method, in common.
First, the redundancy of the reset delays the start of an optical response of the liquid crystal after the reset, or causes an abnormal optical response before starting a normal optical response. This is because a direction, to which the liquid crystal should operate at the response, is not clear at a point in time when the liquid crystal shifts from a predetermined alignment state realized by the reset to the normal response. Therefore, the liquid crystal responds unevenly and unstably. FIG. 6 shows an example of the abnormal optical response. As shown in FIG. 6, the redundancy of the reset causes delay and display abnormality.
The lack of the reset, on the other hand, may cause a situation that the same transmittance cannot be obtained even if the same data is written for a plurality of times in the reset method. When the reset is insufficient, the liquid crystal does not completely become the predetermined alignment state at the reset. Thus, transmittance in accordance with a history of previous frames is shown at a response after the reset. As a result, the one-to-one correspondence between the applied voltage and the transmittance does not hold. Therefore, a desired gray level may not be obtained, or the luminance may be largely different even if the same gray level is displayed.
A sixth problem is that it is difficult to obtain stable display over a wide temperature range. This is because the response speed of the liquid crystal largely depends on temperature. Especially in the reset method and the method disclosed in the Japanese National Publication No. 2001-506376, the foregoing redundancy and lack of the reset significantly occur when the temperature changes. As a result, for example, the luminance significantly decreases at low temperatures. At high temperatures, on the other hand, the response speed between gray levels is increased, and the luminance increases on the whole. Therefore, display gets near the white display, and hence phenomena in which, for example, the whole display becomes whitish.